The invention relates to “active implantable medical devices” as defined by Directive 90/385/EEC of 20 Jun. 1990 of the Council of the European Communities, particularly to devices that continuously monitor heart rhythm and deliver, if needed, electrical stimulation, resynchronization and/or defibrillation pulses to the heart in case of arrhythmia detected by the device. It relates more particularly, but is not limited to, the explantation of those devices which are provided at the distal end with an anchor member such as a helical screw, axially extending from the body of the device and intended to enter the heart tissue by screwing at the selected implantation site.
The invention relates especially, but is not limited to, those devices that are in the form of an autonomous capsule intended to be implanted in a heart chamber (atrium or ventricle, right or left). These capsules are free of any mechanical connection to an implanted (such as the housing of the stimulation pulse generator) or non-implanted (external device such as a programmer or a monitoring device for remote patient monitoring) main device. For this reason these capsules are called “leadless capsules,” to distinguish them from the electrodes or sensors disposed at the distal end of a conventional probe (lead), which is traversed throughout its length by one or more conductors galvanically connecting the electrode or sensor to a generator connected to an opposite, proximal end of the probe. Note, however, that the autonomous nature of the capsule is not inherently a mandatory feature of the invention.
The explantation of these autonomous capsules is a particularly delicate operation because it is necessary, first, to manage to capture the body of the capsule using an explantation accessory and, second, to exercise in this body a torque for detaching it from the implantation site wherein it was retained by the anchoring screw. This unscrewing torque must be large enough to overcome resistance and adhesions resulting from the formation of fibrous tissue at the location of the implantation site.
In the case of endocardial capsules (that is to say capsules attached to the inner wall of an atrial or ventricular chamber, as opposed, for example, to the epicardial capsules secured to the outer wall of the heart), the explantation constraints are increased due to, first, the need to go through the peripheral venous system to introduce the explantation accessory and, second, the need to remove the capsule after it was caught and unscrewed while ensuring its withdrawal through the tight curves of the venous system. These maneuvers must be performed both accurately and in a completely secure method.
Some explantation accessories designated as “lassos” or snares are known, and are commonly used to capture and remove medical devices such as a probe body, defective catheters, guides, etc., out of the heart chambers or out of the venous system. These lassos may include a flexible wire terminated at its distal end by a deformable loop of shape memory metal, the loop extending in the free state in a plane generally perpendicular to the wire plane which supports it. The wire is introduced into the distal opening of a catheter, crossing through it to emerge proximally. The tension of the wire from the proximal end of the catheter has the effect, at the other end, to pull on the loop while progressively making it enter into the catheter wherein it will be housed.
The catheter is introduced into the patient's body, with the fully folded loop in the distal end region. The loop is then deployed from the catheter by pushing the wire from the proximal end. Because of the shape memory of the metal, the loop then recovers its inclined lasso shape relative to the direction of the wire and of the catheter. The lasso can be oriented at will to capture the element to be extracted. Pulling on the wire then allows to partially enter the loop in the catheter, which has the effect of reducing the size and thus to ensure clamping of the element to be removed.
These accessories have the advantage of having a small introduction diameter (from about 2 to 6 Fr, 0.66 to 2 mm), while having a high capture diameter (typically about 10 to 30 mm).
However, in the envisaged application including unscrewing a device such as a capsule screwed into a wall, these accessories are not suitable because they do not transmit significant torque, while to extract a screwed capsule it is necessary to exercise up to 1 N·cm to this capsule. In addition, this unscrewing torque must be substantially exerted in the axis of the capsule.
However, with a conventional lasso accessory, the catheter used to control the size of the loop of the lasso tends to move perpendicular to the element captured by the lasso (that is to say that the axis of the element is oriented perpendicularly to the direction of the catheter), at best parallel to a generatrix of the element. However, in such a configuration any unscrewing action would cause a twisting of the tissue around the fastening screw, without a significant unscrewing effect and with a high risk of tamponade.
Finally, this capturing mode wherein the elongated body of an autonomous capsule would be oriented perpendicular to the catheter axis (the assembly thus taking the form of a T) would be completely incompatible with fully secured retrieval across the venous network.
WO 2012/082755 A1 discloses an explantation accessory for leadless capsule, including one or more lassos capturing a body formed in the posterior portion of the capsule, such as a button, or projecting spouts or hooks. The docking body of the catheter includes a receptacle which fits on the rear portion of the capsule after the latter has been captured by the lasso(s), so as to transmit unscrewing torque required for explantation. This structure, however, is relatively mechanically complex, and furthermore it does not solve the difficulty of the capture maneuver by the lasso, which is to capture very small size spouts or hooks.
US 2009/0163926 A1 describes a lasso catheter for explantation of angeiology devices such as filters inserted in veins, wherein the problem of transmission of a relatively important unscrewing torque does not arise at all.